As a rule, the devices which perform operations upon analog signals such as, for instance, amplifiers, comparators, etc., add a DC voltage component to the signal under process, thereby bringing about a zero level offset. This offset is often the source of errors. Such errors are all the more important in effect when the signal level is low. In addition, when the signal has to be processed throughout an assembly of devices, which is the case when an analog signal is to be converted into a digital signal in a conversion chain, each device introduces a zero level offset which results in an important global error. The value of the output signal of the assembly in such instances does not have the desired relationship with the input signal.
A conventional analog to digital conversion chain is comprised of a plurality of devices. A sample and hold device takes samples of the signal to be converted and stores their values throughout the conversion time period. A comparator receives the value of the stored sample at one input and successively receives reference levels with which the value stored during the conversion time period is compared at the other input. These reference levels can be generated in different ways. They can, for instance, be provided by a digital-analag converter (DAC) under the control of a logic circuit which, according to the result of the comparison, causes the bit pattern converted by the DAC to be varied in order to obtain either a higher or lower reference level. Such a conversion chain is well known in the prior art and is disclosed in the book entitled "Analog to Digital/Digital to Analog Conversion Techniques" by David F. Hoeschle, Jr., issued by John Wiley and Sons, Inc., page 360.
Each of the devices in the conversion chain, namely the sample-hold device, the comparator and the reference level generator, introduces a zero level offset and, therefore, the digitally-coded signal does not correspond exactly to the analog input value.
A solution to this problem consists in making a zero measurement periodically which gives the offset value introduced by the conversion chain. This measurement is made by applying a zero level signal to the input of the conversion chain and coding it. The coded value is the zero offset measurement utilized to make the correction. This value, therefore, must be subtracted from the output signal. This solution has two drawbacks. First, it is not possible to make a dynamic correction of the zero offset. In addition, time must be allowed for the zero measurement, thereby decreasing the conversion rate.